An image forming apparatus is usually provided with a fixing device for fixing a toner image on a recording medium by heat and pressure. For example, a fixing roller having a heater inside and a pressure roller are provided to form a nip. When a recording medium passes through the nip, a toner image is heated by the heater through the fixing roller, and fixed onto the recoding medium by a pressure generated at the nip.
Recently, to reduce a warm-up time, a fixing roller having a low heat capacitance is provided with an external heater. The external heater heats up the surface of the fixing roller, which constantly rotates, at a position away from the nip.
However, the heat applied to the fixing roller may be transmitted to the other member, such as the pressure roller in contact with the fixing roller, thus causing a large amount of energy loss.
Further, the rotation of the fixing roller may accelerate wear of the surface of the fixing roller, or it may increase the amount of electric consumption.